1. Field of the Invention
This invention relates to a process for producing premium coke suitable for manufacture of graphite electrodes for use in electric arc furnaces used in the steel industry.
The electric arc steel making process utilizes large graphite electrodes, having a diameter of from 0.5 to 1 meter, and in order for such electrodes to stand up to this severe use, it is necessary that they be formed from coke of the type generally designated "premium".
Premium delayed petroleum coke has been manufactured for many years and has been called by many names. For example, premium petroleum coke has been called No. 1 coke, electrode grade coke, needle coke, and premium coke. Premium coke is ground, calcined, mixed with a binder pitch, extruded, baked and then graphitized to form graphite electrodes. Premium coke can be distinguished from ordinary, regular, or No. 2 petroleum coke by its predominant metallic, striated, crystalline appearance and its tendency to break into long, splintery, acicular particles when crushed to a fine size. Regular or ordinary petroleum coke is more or less amorphous, has a dark, spongy appearance and breaks into lumps of irregular shape. Premium coke has a low linear coefficient of thermal expansion (CTE). There are several ways of determining and expressing this characteristic. A CTE of 7.0 .times. 10.sup.-7 /.degree. C. or less as measured by the optical lever method on a graphite rod over a temperature range of from 30 to 98.degree. C. is generally required to produce a satisfactory electrode. A CTE of 5.0 .times. 10.sup.-7 /.degree. C. or less is much preferred, particularly for larger diameter electrodes. As used herein, the term premium coke means delayed petroleum coke having a CTE as defined above of not more than 7.0 .times. 10.sup.-7 /.degree. C.
2. Description of the Prior Art
The principal raw materials used for the production of premium grade coke in the past have been carefully selected highly aromatic petroleum refinery residue streams obtained from catalytic and thermal cracking processes. Virgin reduced crude oil has been considered unsuitable as a feedstock for manufacture of premium coke, and in fact such virgin reduced crudes are unsuitable in almost all instances.
U.S. Pat. No. 2,745,794 discloses a combination refinery process including coking, thermal cracking and catalytic cracking. The coke produced in the process is regular grade coke.
U.S. Pat. No. 2,775,549 to Shea discloses an early process for making premium coke from certain petroleum residues.
In U.S. Pat. No 2,922,755 to Hackley, a process is disclosed wherein reduced crude can be mixed with thermal tar to produce a mixture which results in a premium grade coke upon carrying out the delayed coking process provided that this reduced crude is present in the weight percent range of about 10 to about 30.
An article by K. E. Rose in Hydrocarbon Processing, July 1971, pp. 85-92, discusses delayed coking in general, and in particular describes certain coking feedstocks for both premium and regular coke.
In the past, premium coke has been prepared by operating the process on a blocked-out operation. Blocked-out refers to operation of any portion of a process using steams coming from or going to a storage tank. The thermal tar produced from regular coker gas oils is segregated and charged separately to a premium coker on blocked-out operations to make premium grade coke wherein the charge to the coker is primarily thermal tar produced by thermal cracking of a regular coker gas oil stream.
A process for the simultaneous manufacture of regular and premium coke is described in U.S. Pat. No. 3,472,761 to Cameron.
A typical process for making regular coke is described in an article by Kasch et al in the Jan. 2, 1956 issue of The Oil and Gas Journal.